Low poly mesh interrupting normal map in render
There is only so much a normal map can do, see for example this tutorial for an in depth explanation of how it works and what the limitations are. TLDW; changes to the silhouette cannot be represented in a normal map. For situations like your second example a normal map cannot give any close approximation of the original appearance. You could try vector displacement instead if you want.

That said, your first example should be doable with a normal map.


Make sure your low poly object is smooth shaded before baking.
Try adding a little geometry to sort of follow the contour of the high poly mesh (even simply beveling will help reduce terminator artifacts)